How to determine the equations of lines of a square located in the 3rd quadrant?

Answer 1

Depends on what you want the square to look like. A "diamond"-looking square is probably the easiest to characterize:
First quadrant, y=co - x x=0 to x=co
Second quadrant, y=co + x x=-co to 0
Third quadrant, y= -co + x x=-co to 0
Fourth quadrant y = - co + x x=0 to co

Answer 2

hi

3rd quadrant : x<0; y<0

1)x = -a
2)x = -a - l
3)y = -b
4)y = -b - l

bye